Chapter 6

Errors, Error Detection, and Error Control

Topics

Types of noise Error prevention Error detection

Simple parityLongitudinal parityCyclic redundancy checksum (CRC)

Error control

Introduction

Noise is always present. If a communications line experiences too much

noise, the signal will be lost or corrupted. Communication systems should check for

transmission errors. Once an error is detected, a system may

perform some action. Some systems perform no error control, but

simply let the data in error be discarded.

Types of Noise

White Noise Thermal or

Gaussian noise. Relatively constant. Disrupt the signal if

too strong. Use signal

regenerator or filter to remove it

Types of Noise

Impulse Noise Non-continuous One of the most

disruptive ones Destroy one or

more bits of information

Difficult to detect and remove

Types of Noise

CrosstalkUnwanted coupling between two different

signal paths.For example, hearing another conversation

while talking on the telephone.Relatively constant and can be reduced with

proper measures

Types of Noise

Echo The reflective

feedback of a transmitted signal

Most often occurs on coaxial cable.

Could interfere with the original signal.

Relatively constant, and can be significantly reduced.

Types of Noise

Jitter The result of small timing

irregularities Digital signals Occurs when repeated

over and over Could force systems to

slow down their transmission.

Types of Noise

Delay Distortion Occurs because the velocity of propagation of a

signal through a medium varies with the frequency of the signal

Can be reduced

Attenuation The continuous loss of a signal’s strength as it

travels through a medium

Error Prevention

Error Detection Techniques

Despite the best prevention techniques, errors may still happen

To detect an error, something extra has to be added to the data/signal.

Two techniques commonly used:

Simple Parity Checking

If performing even parity, add a parity bit such that an even number of 1s are maintained.

If performing odd parity, add a parity bit such that an odd number of 1s are maintained

Examples send 1001010 using even parity

send 1001011 using odd parity

Longitudinal Parity Checking

Longitudinal parity adds a parity bit to each character then adds a row of parity bits after a block of characters.

The row of parity bits is actually a parity bit for each “column” of characters.

The row parity bits plus the column parity bits add a great amount of redundancy to a block of characters

Catch of errors

What if the following?

Cyclic Redundancy Checksum (CRC) The CRC error detection method treats the packet of data to

be transmitted as a large polynomial. The transmitter takes the message polynomial and using

polynomial arithmetic, divides it by a given generating polynomial.

The quotient is discarded but the remainder is “attached” to the end of the message.

The message (with the remainder) is transmitted to the receiver.

The receiver divides the message and remainder by the same generating polynomial.

If a remainder not equal to zero results, there was an error during transmission.

Cyclic Redundancy Checksum (CRC Cost

Performance

Error Control

Once an error is detected, what is the receiver going to do?

1. Do nothing (Frame Relay)

2. Return an error message to the transmitter and request for resend

Stop-and-Wait ARQ Go-Back-N ARQ Selective-Reject ARQ

3. Fix the error with no further help from the transmitter